Bridge demodulator circuit



0. O. FIET BRIDGE DEMODULATOR CIRCUIT 2 Sheets-Sheet 2 INVENTOR We]! 0life? BY M w ATTORNEY March 29, 1955 Original Filed Feb. 1, 1952 UnitedStates Patent BRIDGE DEMODULATOR CIRCUIT Owen 0. Fict, Oaklyu, N. J.,assiguor to Radio Corporation of America, a corporation of DelawareContinuation of abandoned application Serial No. 269,541. Thisapplication April 19, 1954, Serial No. 424,033

Claims. (Cl. 332-47) This invention pertains to an improved demodulatingsystem and more particularly to a monitor system wherein doubleheterodyning is employed utilizing two bridgetype or balanced mixercircuits.

This application is a continuation of copending application Ser. No.269,541, filed on February 1, 1952, now abandoned.

In transmitting television signals from a broadcast transmitter, it isdesirable to have a demodulator capable of producing a very high qualityvideo signal representative of all of the picture energy radiated by theantenna system of the broadcast transmitter. For monitoring purposes,the video information impressed on the carrier wave should not bedistorted by the demodulating equipment. For this reason ordinarytelevision receivers suitable for home use are not desirable as stationmonitors because of the possibility of local oscillator drift, impairedvideo passband characteristics, and perhaps a variable attenuation ofthe different frequencies contained in the video channel to which theyare tuned.

The present invention provides a high fidelity demodulation system whichis extremely stable in operation and which produces an intermediatefrequency signal containing a true representation of the information ofthe transmitted carrier wave. The demodulator system of the inventionwill be described particularly with respect to a monitoring system foruse with broadband very high and ultra-high frequency signals suitablefor television picture transmission; but in its broadest aspects, thedemodulator is utilizable for other types of signal modulation systems.

Among the objects of the present invention are: to provide a noveldouble heterodyne demodulator for producing an extremely stableintermediate freequency wave containing a faithful representation of themodulation of the carrier wave; to provide a novel monitoring system foruse at the transmitting location which faithfully reproduces theintelligence signal information impressed on the transmitted carrierwave: to provide a novel bridge-type mixer arrangement; and to providean improved structural arrangement of a bridge mixer suitable for use atultra-high frequencies.

In accordance with the present invention, there is provided ademodulation system which employs a first mixer circuit to combine asample of unmodulated carrier frequency with a stable offset frequencyoscillator. The output of this first mixer may then be filtered toremove unwanted frequency components. A second mixer circuit is of thebalanced bridge type and combines a sample of the modulated carriercontaining the information to be transmitted with the filtered output ofthe first mixer. The output of the second mixer contains only the offset frequency plus the modulation information, and may be detected anddisplayed in a conventional manner.

A feature of the invention resides in the structural arrangement of thebridge-type ultra-high frequency mixer circuits which utilize coaxiallines. The coaxial lines forming part of the bridge-type mixers properlyterminate the system with. respect to each of the several signal sourcesto minimize reflections and at the same time compensate for theintroduction of reactance from other circuit elements.

A more detailed description of the invention follows in conjunction witha drawing wherein:

Figure l is a block diagram of the demodulation systern of theinvention;

Figure 2 is an electrical circuit diagram of the invention arranged foroperation in the very high frequency range;

Figure 3 is a cross-sectional view of the demodulator of the inventionemploying coaxial lines for use at ultrahigh frequencies;

Figure 4 is a cross section in elevation of the same demodulator takenalong the line IV of Figure 3;

Figure 5 is a cross section of one portion of one of the1 mixer circuitstaken along the line V of Figure 4; an

Figure 6 is a schematic representation of one portion of an alternativearrangement of one of the mixers shown in Figure 3.

Referring to Figure 1, there is shown a block diagram illustrating theoperation of the demodulation system of the invention utilizing doubleheterodyning by means of two bridge-type mixer circuits. That portion ofthe diagram to the left of the vertical dash line 10 represents aconventional transmitter, while that portion to the right of the line 10represents the demodulator or monitoring system of the invention. Afirst mixer A shown by box 11, which may be a balanced bridge-typemixer, is fed with a sample of a stable unmodulated carrier, for examplefrom the carrier frequency oscillator 13 of a radio or televisiontransmitter. Mixer A is also fed from a separate controlled source ofoscillations 15 of an offset frequency f0, which may be a desired outputintermediate frequency upon which the modulation of the transmitter isto be impressed for monitoring purposes. The output of the bridge-typemixer A consists of both sidebands, foifo. The carrier frequency in aswell as the offset frequency Q is suppressed in the output of thebridge-type mixer A. The output of mixer A is then passed thru a filter17 to substantially eliminate the offset carrier 1% and an undesiredsideband, for example fc+fo. This leaves at the output of the filter 17substantially only a single frequency component, for example fc-fo.

The first mixer A need not be either a balanced or a bridge'type mixer.An ordinary single-sided heterodyne mixer may be employed in the firstmixing action. A bridge-type or balanced mixer is preferable for manyapplications since one or both of the frequencies which are fed to sucha mixer are suppressed in the output of the mixer. Unless a balancedmixer or bridge-type mixer is used, additional filtering may benecessary in the filter 17 to eliminate all of the undesired mixerproducts.

From the same carrier frequency oscillator 13 from which one input tothe mixer A was derived, energy is taken, amplified and applied to amodulator circuit 19 where it is modulated by intelligence signal energyIn from a source 21, which may be, for example, voice, telegraph,picture, or similar signal information. The output of the modulator 19depends upon the type of modulation employed and may be, for example,single sideband with carrier suppression, both sidebands with carriersuppression, both sidebands with carrier, or carrier plus one sidebandin its entirety and only part of the other sideband. The last type ofmodulation, carrier plus one sideband complete and only part of theother sideband, is termed vestigial sideband modulation and is presentlyemployed in television picture transmission.

The output of the modulator 19 may be amplified by a conventionalamplifier stage 23 to bring it to the proper level for transmission tothe utilization circuit, such as a transmission line or a broadcastantenna or the like. The amplifier stage 23 is unnecessary in the typeof transmitter in which the final output stage is modulated. With eithersystem, the modulated carrier fail?! is coupled from a final modulatedamplifier or an amplifier stage 23 to an output circuit 25. such as anantenna or transmission line.

A bridge-type second mixer 27 designated as mixer B serves to combinethe desired output of mixer A with a sample of the modulated carrierfrom the amplifier stage 23. By utilizing a bridge-type or balancedmixer B to combine the two signals, the carrier frequency fc is balancedout and the filtered mixer product of mixer A, for example, fc-fo, issuppressed. The output of the mixer B then contains the desired oifsetcarrier frequency in and the modulation products originally impressed onthe carrier wave (him), but now impressed on the offset frequency wave,that is him.

The output of the mixer B is then fed to an offset frequency amplifier29 which may be, for example, a broad tuned amplifier such as anintermediate frequency amplifier. For television picture demodulation,the offset frequency amplifier 29 is preferably tuned to one of thestandard intermediate frequencies for televislon reception, for example30 megacycles. As has been mentioned, the offset oscillator 15 producesoscillations of the same intermediate frequency, in this case 30megacycles.

When the demodulator of the present 1nvent1o n is utilized fortelevision picture demodulation, provision must be made to suppress anycoupling from the output of the offset frequency oscillator 15 into theoffset frequency amplifier 29. This is necessary because any amount ofthe offset frequency wave would be amplified in the offset frequencyamplifier 29 and demodulated as a direct current component. The presenceof even a small amount of stray energy having a frequency WhlChrepresents the direct current component (is) at the offset frequencyrenders the demodulator practically useless for monitoring purposes.With an unknown amount of such energy, the monitoring apparatus will notaccurately measure the percentage modulation of the carrier Wave and cannot be relied upon to give a true indication of the direct currentcomponent or the various synchronizing and blanking levels which must bemaintained within extremely rigid tolerance.

One method of suppressing the coupling from the output of the offsetfrequency oscillator 15 is to connect that oscillator across thebridge-type mixer A in such a way that the offset frequency f issuppressed in the output of the mixer A. Another arrangement is toutilize circuit elements in the filter 17 which effectively shortcircuitand effect an extremely high loss to energy at the offset frequency f0.

After being amplified in the offset frequency amplifier 29, themodulated offset frequency signals him are supplied to a conventionaldetector 31 where they are treated in the usual manner and may bedisplayed in the case of a visual signal, or converted to either anaural or visual signal in case of sound information, for monitoringpurposes.

When the demodulation system explained in connection with Figure 1 isemployed, it will be seen that the output of mixer B will contain nodistortion due to frequency drift or shift of the carrier frequencyoscillator 13. By combining a sample from the carrier frequencyoscillator 13 with the offset frequency oscillator 15, the output ofmixer A will change in frequency by the same amount as the sample ofmodulated carrier, him, which is supplied to mixer B. When the twosignals, one from mixer A and the sample from the amplifier stage 23 areapplied to the mixer B, no distortion or alteration of the passband ispresent at the input of the offset frequency amplifier 29. The onlycritical oscillator, as far as the bandpass characteristics of theoffset or intermediate frequency amplifier 29 are concerned, is theoffset frequency oscillator 15. This offset frequency oscillator 15operates at a lower frequency than a local oscillator in a conventionalsuper-heterodyne demodulation system and therefore can be made much morestable, for example, by being crystal controlled, to insure that theband of frequencies to be detected will be properly placed in thepassband of the offset or intermediate frequency amplifier 29.

Referring now to Figure 2, there is shown a circuit diagram of thedemodulation system of Figure 2 adapted for operation in the very highfrequency range. A bridgetype crystal mixer is shown within the dottedline box 11 and corresponds to mixer A of Figure 1. The bridgetypecrystal mixer 11 is supplied with a sample of the transmitter carrierfrequency from the carrier frequency oscillator 13 in push-pull by meansof a carrier input transformer 33 which is center-tapped to form twoarms of a bridge. The output from the offset frequency oscillator 15 iscoupled across a diagonal of the bridge-type crystal mixer 11 by meansof an offset carrier input transformer 35. Two nonlinear conductingdevices 37, 39 comprise the remaining two arms of the bridge and will bedescribed hereafter as crystal rectifiers, although it is to beunderstood that other nonlinear devices, such as diodes, triodes, andmultigrid tubes, as well as dry rectifiers such as copper oxide andselenium, may also be employed. The output of the bridge-type crystalmixer 11 is taken between the junction of the crystal rectifiers 37, 39and a point of potential reference 40 such as ground. Due to thebridge-type mixer arrangement, energy from the source which is connectedacross the two arms of the bridge formed by the input transformer 33does not appear in an output which is taken across the diagonal of thebridge. The sum and difference frequencies of the two sources arefull-wave rectified and appear as the main output across the diagonal ofthe bridge between the junction of the crystal rectifiers 37, 39 and thepoint of potential reference 40.

The output of the first bridge-type crystal mixer 11 containing the sumand difference components of the carrier frequency oscillator 13 and theoffset frequency oscillator 15 (fcifo) may then be filtered to removeundesired frequency components. A filter 17 is shown which is designedto have one arm series resonant by tuning an inductance 41 and acapacitor 43 to an undesired frequency component, for example fc-l-fo,or to both ft: and fc-i-fo. To give the required attenuation to theoffset frequency f0, another arm composed of an inductance 42 and acapacitor 44 may be added and series tuned to the offset frequency 1%.The entire filter 17 may then be made parallel resonant to the desiredfrequency component, for example fcfo by adding a third arm 45 which hasthe proper reactance at the desired frequency. The total reactance ofthe two series-tuned arms, one composed of the inductance 41 and thecapacitor 43 and the other composed of the inductance 42 and thecapacitor 44, may be either capacitive or inductive at the lowerfrequency product fcfo. If this total reactance is capacitive, aninductive reactance is inserted as the third arm 45 to make the entirefilter 17 parallel resonant at the desired frequency fcjo. Similarly, ifthe total reactance of the two arms is inductive at the desiredfrequency fcfo, a capacitive reactance would be inserted in the place ofthe inductive reactance shown as the third arm 45 to make the entirefilter 17 parallel resonant at the desired frequency.

Depending upon the range of frequencies employed and the ratio ofdifference in frequency between the carrier frequency f and the offsetfrequency f0, it may be necessary to couple the offset frequencyoscillator 15 across the two arms of the bridge and apply the signalfrom the carrier frequency oscillator 13 across the diagonal of thebridge. This will result in a better suppression of the offset frequencysignal, which, as mentioned above, is necessary when percentagemodulation of the carrier wave and the various synchronizing andblanking levels of a television picture signal are to be accuratelymeasured. Theoretically, with the bridge type of mixer cir cuit, energywhich is coupled across the arms of the bridge is completely suppressedin an output taken across a diagonal. Actually, however, such perfectbalance is difficult to attain. It becomes desirable therefore that thefilter 17 be designed to have additional rejection by means of a seriesresonant arm to the energy which is coupled across the center-tappedinput transformer 33.

A second bridge-type crystal mixer 27 is supplied with a sample of themodulated carrier frequency from the final amplifier 23 of thetransmitter across a centertappcd Winding of a signal input transformer47 which forms two arms of a bridge. The remaining two arms of thebridge are composed of two nonlinear conducting devices 49, 51 similarto those described above in connection with the bridge-type crystalmixer 11, and will be referred to hereafter as crystal rectifiers. Theoutput of mixer 11, after being filtered, for undesired components inthe filter 17, is therefore impressed between the junction of thecrystal rcctifiers 49, 51 and the point of reference potential 40. Themain output across the diagonal of the bridge is the difference betweenthe filtered output of the first mixer 11 and the sample of modulatedcarrier fcim from the amplifier stage 23. This output faint comprisesthe modulation m originally present on the carrier wave, that is, fcztm,but now imposed on the offset frequency wave.

Referring now to Figures 3 and 4, Figure 3 shows a cross-section inelevation and Figure 4 is a cross-section end view of the double mixerdemodulator of the present invention employing coaxial lines for use atultra-high frequencies. In this embodiment, a first bridge-type mixerincludes a coaxial arrangement having an inner conductor 61, anintermediate cylindrical wall 63, and

an outer cylindrical wall 65. A source of carrier frequency such as acarrier frequency oscillator 13 is coupled between the inner conductor61 and the intermediate cylindrical wall 63. The inner conductor 61 andthe intermediate cylindrical wall 63 constitute a coaxial transmissionline and is preferably proportioned to match the impedance at the outputof the carrier frequency oscilla tor 13.

The outer cylindrical wall 65 is short-circuited by an annular shortingbar 69 to the intermediate cylindrical wall 63 at a point removed fromthe end of the intermediate cylindrical wall 63. The end of theintermediate cylindrical wall 63 farthest removed from the annularshorting bar 69 is bifurcated for a distance of approximatelyone-quarter wavelength at the carrier frequency fc to provide twoparallel conductors. The arrangement of the bifurcated end of theintermediate cylindrical wall 63 is illustrated further in connectionwith Figure 5 and will be additionally described below in connectionwith that figure. The bifurcated portion of the intermediate cylindricalwall 63 is capacitively coupled to a similarly bifurcated sleeve 71which fits within the bifurcated end portion of the intermediatecylindrical wall 63. The bifurcated sleeve 71 is spaced from theintermediate cylindrical wall 63 by a cylindrical sleeve of insulation73.

A source of offset frequency f0, for example from the offset frequencyoscillator 15, is coupled between the outer cylindrical wall 65 and thebifurcated'sleeve 71. When the source of offset frequency 15 is remotelylocated with respect to the mixer, a coaxial cable 67 may be utilized.Such a coaxial cable 67 would have its outer conductor directlyconnected to the outer cylindrical wall 65 and its inner conductordirectly connected to the bifurcated sleeve 71. It is desirable in manyapplications of the present invention to locate the offset frequencyoscillator 15 directly on the outer cylindrical wall 65 and provide goodshielding to prevent energy from the offset frequency oscillator frombeing coupled thru stray fields outside of the mixer structure to otherportions of the demodulator circuit, such as for example the offsetfrequency amplifier shown in Figure 1. With this arrangement, the outputof the offset frequency oscillator 15 would be single-sided with respectto the outer cylindrical wall 65, and an inner conductor correspondingto the inner conductor of the coaxial cable 67 would extend thru anaperture in the outer cylindrical wall 65, an aperture in theintermediate cylindrical wall 63 and the cylindrical insulating sleeve73 and be directly connected to the bifurcated sleeve 71. w

The inner conductor 61 has its end coupled at radio frequencies to oneside of the bifurcated sleeve 71. As shown in Figures 3 and 4 of thedrawing, this radio frequency connection may be made thru a capacitivecoupling which is composed of a tubular sleeve 75 of insulation and aconductive tube 77 surrounding the tubular insulating sleeve 75 and theend portion of the inner conductor 61. The conductive tube 77 iselectrically connected to one side of the bifurcated sleeve 71 by ablock of conductive material 79. The block of conducting material 79 maybe shaped as a semicircle to fit into one side of the bifurcated sleeve71 and the other side made straight to make contact with the conductivetube 77. At frequencies above 300 rnegacycles it will be found thatintroducing such a block of conductive material on one side of abifurcated sleeve line will create a slight unbalance and reflection.For this reason it is desirable to include a similar semicircular block80 at the end portion of the other part of the bifurcated sleeve 71. Ofcourse, the second similar semicircular block 80 will not beelectrically connected or coupled to the inner conductor 61. Thearrangement of this portion of the circuit can best be visualized froman inspection of Figure 4.

Mechanically secured to and electrically connected to each of the twoends of the bifurcated sleeve 71 is a pair of unilateral conductingdevices 37 and 39 like those described in connection with Figure 2. Dueto their small size and excellent electrical characteristics, crystaldiode rectifiers are preferred in the structure of this invention,although other types of nonlinear conductive devices may be utilized.Both crystal rectifiers 37 and 39 are shown as poled to be conductivetoward a metallic ring 81 and are electrically connected andmechanically secured thereto. The operation of the present inventionwould not be altered if both crystal rectifiers 37 and 39 were reversedto be conductive away from the metallic ring 81 and toward the two endsof the bifurcated sleeve 71. Also, if one of the two crystal rectifiers37 or 39 is reversed, mixing action still takes place and all of thedesired mixer frequencies (fcifo) are obtained, but in addition thebridge circuit is symmetrical both with re-- spect to the source coupledacross the inner conductor 61 and the intermediate cylindrical wall 63,and to the source coupled in push-push between the intermediatecylindrical wall 63 and the outer cylindrical wall 65. This latterarrangement is preferable where suificient filtering is diflicult toobtain.

A bridge-type second mixer is also provided to combine the output of thebridge-type first mixer just described with a sample of the modulatedcarrier from a source 23 which may be a final amplifier like theamplifier stage 23 described above in connection with Figures 1 or 2.The sample of modulated carrier may also be derived from thetransmission line to the utilization device, for example, thetransmission line between the transmitter amplifier and an antenna inthe usual broadcast station installation. For very high and ultra-highfrequency applications, especially where: television picturetransmission is being monitored, it is preferable to sample themodulated carrier him in the transmission line between the transmitteroutput and the antenna. A directional coupler may be employed for thispurpose. Such directional couplers are sensitive to energy flowing inonly one direction in the transmission line and give an accurate sampleof the energy flowing toward the antenna and radiated thereby. Such adirectional coupler which may be used as the source 23' in thisinvention is disclosed and claimed in my copending application SerialNo. 269,542, filed on February 1, 1952, and assigned to the sameassignee as this application.

The bridge-type second mixer also includes a coaxial arrangement havingan inner conductor 91, an intermediate cylindrical wall 93 and an outercylindrical wall 95. The source of modulated carrier 23 is coupledbetween the inner conductor 91 and the intermediate cylindrical wall 93.As in the first mixer, the inner conductor 91 and the intermediatecylindrical wall 93 constitute a coaxial transmission line proportionedto match the impedance of the source of modulated carrier frequency 23.

The output of the first mixer is coupled between the outer cylindricalwall 95 and the intermediate cylindrical wall 93 by means of the jointbetween the two outer cylindrical walls 65 and 95 of the mixers and themetallic ring 81 which is mechanically secured to and makes electricalcontact with the intermediate cylindrical wall 93 of the second mixer.

In the second mixer, the outer cylindrical wall 95 is short-circuited byan annular metallic shorting bar 99 to the intermediate cylindrical wall93 at a point removed from the metallic ring 81. The end of theintermediate cylindrical wall 93 farthest removed from the annularshorting bar 99 is bifurcated for a distance of approximatelyone-quarter wavelength at the carrier frequency Jc to provide twoparallel conductors. The arrangement of the bifurcated end of theintermediate cylindrical wall 93 of the second mixer is illustrated ingreater detail in Figure 6 and will be additionally described below.

The inner conductor 91 of the second mixer is connected to one side ofthe bifurcated end of the intermediate cylindrical wall 93. Thisconnection is made thru a semicircular block 101 similar to thesemicircular block 79 described above in connection with the firstmixer. A second semicircular block 102 is provided which fits into andmakes contact with the other side of the bifurcated end of theintermediate cylindrical wall 93. As explained above in connection withthe first mixer A, this second semicircular block 1102 is not connectedto the inner conductor 91 of the second mixer, but is provided only forbalancing the bifurcated line portion of the intermediate cylindricalwall 93.

Mechanically secured to and electrically connected to each side of thebifurcated end of the intermediate cylindrical wall 93 of the secondmixer are identical crystal rectifiers 49, 51. Each of these rectifiersis poled toward and is electrically connected to a metallic disc 103which is in capacitive relation to the outer cylindrical Wall 95 of thesecond mixer by virtue of its close spacing to the wall 95. Either orboth of the rectifiers 49, 51 may be reversed in polarity in the secondmixer also as explained above in connection with the first mixer.

The metallic disc 103 is electrically connected to a cylindrical innerconductor 105 which together with a continuation of the outercylindrical wall 95 of the second mixer forms a coaxial output line. Theoutput of the second mixer thus appears between the cylindrical innerconductor 105 and the outer cylindrical wall 95 single-sided withrespect to the outer cylindrical wall as a point of potential reference.

A coaxial trap portion having an outer conductor 107, an inner conductor109 and an adjustable shorting metallic block 111 is used to accomplishpart of the filter action like the filter 17 of Figures 1 or 2. Formaximum Q, and therefore for maximum loss in the coaxial trap, thediameter ratio of outer to inner conductors should be 3.59 to 3.6.

The coaxial trap is adjusted by setting its length to have its greatestloss at one frequency which it is desired to eliminate. The reactance ofthe trap section. including the outer conductor 107, the inner conductor109, and the shorting block 111, is tuned out at the desired outputfrequency by adjustment of the annular shorting bar 99 in the secondmixer B. The annular shorting bar 69 of the first mixer may be utilizedin conjunction with the coaxial trap to form another circuit forattenuating a different undesired frequency.

In Figure 5, which is a cross section of one portion of the mixer Ataken along a line V of Figure 4, the relation of the outer cylindricalwall 65, the intermediate cylindrical wall 63, and the inner conductor61 can readily be seen. The bifurcated portion of the intermediatecylindrical wall 63 is shown as having a cylindrical sleeve ofinsulation 73 separating each side of the bifurcated portion of theintermediate cylindrical wall 63 from the bifurcated sleeve 71. ductor61 in this cross-sectional portion is shown surrounded by the tubularinsulating sleeve 75 which is itself surrounded by the conductive tube77. While the length of the slot in each side of the bifurcated sleeve71 and intermediate cylindrical wall 63 is direetly related to theoperating wavelength, the width of the slot is effected by bandwidthconsiderations. Generally, the rule is that the wider the slot thegreater percentage bandwidth in each case.

Referring to Figure 6, there is shown an alternative arrangement of oneportion of one of the mixers shown in Figures 3 and 4. In order toobtain a push-pull output from. for example, mixer B of Figure 3 or 4,the single metallic disc 103 of Figures 3 and 4 is replaced with twoseparate discs or semicircular shaped blocks 113 and 115. Each of thesediscs 113 and 115 is balanced capacitively with respect to the outerconductor 95 which forms a shield for the balanced output line. The twoconductors 117, 119 form a balanced output line, are identical, and areproportioned in diameter and spacing from each other and from the outerconductor 95 to have the proper characteristic impedance to match theamplifier or detector with which the mixer output is coupled.

In Figure 6, also the structure of the connection between the innerconductor 91 and the bifurcated portion of the intermediate conductor 93can more easily be seen. It will be noticed that in the vicinity of thesemicircular closing block 101, the inner conductor 91 is cut off on adiagonal so that proper clearance is obtained between the innerconductor 91 and the other semicircular closing block 102.

In the operation of the demodulator of this invention utilizing thecoaxial line crystal mixers at ultra-high frequencies, the first mixer Ais utilized to combine the source of carrier frequency with a highlystable source of offset frequency oscillations. In the arrangement shownin Figures 3 and 4, a convenient method of operation is as follows: Thesource of carrier frequency fc is coupled across the inner conductor 61and the intermediate cylindrical wall 63. The length of the bifurcatedportion of the intermediate cylindrical wall 63 is approximatelyone-quarter wavelength at the carrier frequency, but due to thecapacitive loading near the end of the bifurcated slot portion, thisbifurcated section will be shorter than one-quarter wavelength for afree space wave. The source of offset frequency is coupled between thebifurcated sleeve 71 and the outer cylindrical wall 65.

The differences in frequency between the source of The inner concarrierfrequency oscillator 13 and the offset frequency oscillator 15 may beconsiderable. For example, for certain applications, the carrierfrequency oscillator may operate at 850 megacycles and the offsetfrequency oscillator 15 may operate as mentioned above at 30 megacycles.Due to this large frequency ratio, certain portions of the circuit whichhave appreciable electrical length at 850 megacycles have negligibleelectrical length at 30 megacycles. Likewise, certain circuit elementswhich appear as short circuits for one frequency will appear asextremely high impedances or open circuits for the other frequency.Take, for example, the capacitance formed between the bifurcated portionof the intermediate cylindrical wall 63 and the bifurcated sleeve 71. Tothe 850 megacycle source 13 this capacitance is practically a shortcircuit, but from the viewpoint of the offset carrier oscillator 15operating at 30 megacycles, considerable impedance is presented.

Due to the quarter-wavelength slot in the intermediate cylindrical wall63 and the bifurcated sleeve 71, and the connection of one side of theslot to the inner conductor 61, the carrier frequency voltage 1% appearsacross the crystal rectifiers 37, 39 of the mixer A in phase oppositionor push-pull relationship. The voltage from the offset frequencyoscillator 15 which is coupled between the bifurcated sleeve and theouter wall structure appears across the crystal rectifiers 37, 39 inpush-push relationship. From an inspection of the circuit of Figure 2,it can be seen that the operation of Figure 3 is similar to that ofFigure 2 except that lumped circuit elements are not employed.

The trap portion composed of the outer conductor 107, the innerconductor 109 and the shorting block 111 opposite the first mixer A istuned to resonance at the carrier frequency, in this case 850megacycles. The annular shorting bar 99 of the second mixer B is tunedin conjunction with the trap 107, 109, 111 to pass the desired sideband,in this case the lower sideband of 820 megacycles. The annular shortingbar 69 of the first mixer A may be adjusted to reduce other undesiredfrequencies.

The modulated carrier is coupled into mixer B across the inner conductor91 and the intermediate cylindrical wall 93. The length of the slottedportion of the intermediate cylindrical wall 63 beyond the metallic ring81 to the two semicircular blocks 101, 103 is tuned to be effectiveone-quarter wave at the carrier frequency.

As mentioned in connection with mixer A above, due to the electricallength of the slotted portion of the intermediate cylindrical wall 93and the connection of the inner conductor 91 to one side of thebifurcated portion of the intermediate cylindrical wall 63, themodulated carrier wave fcilTI appears across the crystal rectifiers 49,51 in push-pull relationship. The output voltage from the first mixer A,however, appears between the crystal rectifiers 49, 51 in push-pushrelationship.

The second mixer B, according to this example, combines the desiredlower sideband frequency output of the first mixer A with the modulatedcarrier which has been sampled from the transmitter. Assuming vestigialsideband picture modulation to be present on the transmitted carrierwave, a band of frequencies from 848.5 megacycles to 854.5 megacycleswill be present and impressed in pushpull across the crystal rectifiers49, 51. The lower sideband frequency of 820 megacycles is impressed inpush-push, that is, in equal voltage relation in phase across thecrystal rectifiers 49, 51. The mixer products of the second mixer B,after being rectified, which appear between the metallic disc 103 andthe outer cylindrical wall are as follows: fo+l1l, in this example 28.5megacycles to 34.5 megacycles; fc-fo, in this example 820 megacycles;him, in this example 848.5 to 854.5 megacycles; and certain even higherfrequency components such as 2fcfo, etc. The capacitive coupling betweenthe metallic disc 103 and the outer cylindrical wall 95 serves as anultra-high frequency bypass for the rectified components of frequenciesvary far above the offset frequency f0. For frequencies in the rangeabove the lowest undesired frequency likely to be impressed between themetallic disc 93 and the outer cylindrical wall 95, that is fc-fo andupward, this capacitive coupling is in effect a short circuit. In therange of frequencies desired as an output from mixer B, that is foil",the capacitance between the metallic disc 103 and the outer cylindricalwall 95 offers appreciable impedance.

The arrangement of the present invention offers a method of demodulatingultra-high frequency signals which has numerous advantages overconventional systems for monitoring purposes. The use of a balanced orbridge-type mixer as the second mixer B will prevent spuriousfrequencies generated by the monitoring equipment from being introducedinto the output of the transmitter. The introduction of such spuriousfrequencies into the transmitter can result in unauthorized radiation bythe transmitting antenna of undesirable amounts of spurious frequencysignals.

Another advantage lies in the frequency stability of the presentinvention. Where a local oscillator of a conventional system is used tobeat directly against the transmitted carrier wave to produce anintermediate frequency wave containing the modulation information, theinstability of the local oscillator in such an arrangement producesdistortion in the demodulated wave out of all proportion to thepercentage of instability. Consider, for example, the case where an 850megacycle signal is being transmitted and the local oscillator isoperating at a frequency of 820 megacycles to give a 30 megacycleintermediate frequency wave. When the local oscillator changes infrequency by 1%, the intermediate frequency wave changes in frequency by27.3%. An ultra-high frequency oscillator to beat directly with thetransmitted ultra-high frequency wave which would offer the same degreeof stability in the offset or intermediate frequency wave must thereforebe 27.3 times as accurate as the offset oscillator in the system of thepresent invention. the ultra-high frequency television band from 470 to890 megacycles, such a directly beating oscillator must be from 15 to 29times as accurate in frequency as the offset frequency oscillator of thepresent invention in order to provide the same faithfulness ofreproduction of the modulation information on the transmitted carrierwave. The system of the present invention, by means of its doubleheterodyning feature, overcomes the foregoing difficulty of theconventional systems.

What is claimed is:

l. A demodulation system for signal modulation imposed on a carrierfrequency wave comprising: a first mixer circuit including an innerconductor, an intermediate conductor surrounding said inner conductor,an outer conductor surrounding said intermediate conductor, saidintermediate conductor being slotted for a distance of approximatelyone-quarter wavelength at the carrier frequency f0 to form a bifurcatedend, a radio frequency connection from said inner conductor to one sideof said bifurcated end of said intermediate conductor, means couplingunmodulated carrier frequency energy fc across said inner andintermediate conductors, a source of offset frequency oscillations f0,means coupling said source of offset frequency is between said outerconductor and said intermediate conductor, and two rectifiersrespectively connected to different sides of said bifurcated end of saidintermediate conductor; a second mixer circuit including an innerconductor, an intermediate conductor surrounding said inner conductor,an outer conductor surrounding both said inner conductor and saidintermediate conductor, said intermediate conductor being slotted for adistance of approximately one-quarter wavelength at the carrierfrequency ft: to form a bifurcated end, a connection from said innerconductor to one side of said bifurcated end of said intermediateconductor, means coupling said signal modulated carrier frequency wavefcim across said inner and intermediate conductors, a connection fromeach of said two rectifiers of said first mixer to said intermediateconductor of said second mixer, two additional rectifiers beingrespectively connected to different sides of said bifurcated end of saidintermediate conductor of said second mixer; an output transmission linehaving an outer conductor electrically connected to said outer conductorof said second mixer and having an inner conductor electricallyconnected to both of said additional rectifiers of said second mixer;and a coaxial filter associated with said first mixer tuned to reject energy having a frequency of the sum of said carrier frequency and saidoffset frequency fcifo.

2. A demodulation system for signal modulation in imposed on a carrierfrequency wave in comprising a first mixer circuit, a source of offsetfrequency oscillations is, means coupling said source ft) to said firstmixer, second means coupling unmodulated carrier frequency energ y is tosaid first mixer; a coaxial filter associated with said first mixertuned to reject undesired frequency mixer products; a bridge-type secondmixer circuit including an inner conductor, an intermediate conductorsurrounding said inner conductor, an outer conductor surrounding saidintermediate conductor, said intermediate conductor being slotted at oneend for a distance of approximately onequarter wavelength at the carrierfrequency f0 to form a bifurcated end, a connection from said innerconductor to one side of said bifurcated end of said intermediateconductor, means coupling said signal modulated carrier frequency wavefcim across said inner end and intermediate conductors, a connectionfrom the output of said first mixer to said intermediate conductor ofsaid sec ond mixer, two rectifiers respectively connected to differentsides of said bifurcated ends of said intermediate conductor of saidsecond mixer; and an output transmission line having an outer conductorelectrically connected to said outer conductor of said second mixer andhaving an inner conductor electrically connected to both of saidrectifiers of said second mixer.

3. A demodulation system for signal modulation m imposed on a carrierfrequency wave fc comprising a first mixer circuit, a source of offsetfrequency oscillations in, means coupling said source ft, to said firstmixer, second means coupling unmodulated carrier frequency energy fc tosaid first mixer; a coaxial filter associated with said first mixertuned to reject undesired frequency mixer products; a bridge-type secondmixer circuit including an inner conductor, an intermediate conductorsurrounding said inner conductor, an outer conductor surrounding saidintermediate conductor, said intermediate conductor being slotted at oneend for a distance of approximately onequarter Wavelength at the carrierfrequency fc to form a bifurcated end, a connection from said innerconductor to one side of said bifurcated end of said intermediateconductor, means coupling said signal modulated carrier frequency wavefcim across said inner and intermediate conductors, a connection fromthe output of said first mixer to said intermediate conductor of saidsecond mixer, two rectifiers respectively connected to different sidesof said bifurcated end of said intermediate conductor of said secondmixer; and an output transmission line having an outer conductorelectrically connected to said outer conductor of said second mixer andhaving an inner conductor electrically coupled to derive the rectifiedproduct of said second mixer.

4. A mixer circuit for ultra-high and very high frequency wavescomprising an inner conductor, an intermediate conductor surroundingsaid inner conductor, an outer conductor surrounding said intermediateconductor, said intermediate conductor being slotted at one end for adistance of approximately one-quarter wavelength at a carrier frequencyf0 to form a bifurcated end, a bifurcated conductive sleeve incapacitive relation to said bifurcated end of said intermediateconductor, a conductive tube in capacitive relation to said innerconductor, a connection from said conductive tube to one side of saidbifurcated sleeve at the open end thereof, means adapted to couple asource of carrier frequency energy fc across said inner and saidintermediate conductors, additional means adapted to couplea source ofoffset frequency energy in between said outer conductor and saidbifurcated sleeve, two nonlinear devices respectively connected todifferent sides of said bifurcated sleeve, and an output circuit coupledto said nonlinear devices and said outer conductor to derive a rectifiedproduct of said mixer.

5. A mixer circuit for ultra-high and very high frequency wavescomprising an inner conductor, an intermediate conductor surroundingsaid inner conductor, an outer conductor surrounding said intermediateconductor, said intermediate conductor being slotted at one end for adistance of approximately one-quarter wavelength at a carrier frequencyft: to form a bifurcated end, a bifurcated conductive sleeve incapacitive relation to said bifurcated end of said intermediateconductor, a radio frequency connection from said inner conductor to oneSide of said bifurcated sleeve at the open end thereof, means adapted tocouple a source of carrier frequency across said inner and saidintermediate conductors, additional means adapted to couple a source ofoffset frequency between said outer conductor and said bifurcatedsleeve, two nonlinear devices respectively connected to different sidesof said bifurcated sleeve, and an output circuit coupled to saidnonlinear devices and said outer conductor to derive a rectified productof said mixer.

6. A demodulation system for signal modulation m imposed on a carrierfrequency wave f comprising: a first mixer circuit including an innerconductor, an intermediate conductor surrounding said inner conductor,an outer conductor surrounding said intermediate conductor, saidintermediate conductor being slotted for a distance of approximatelyone-quarter wavelength at the carrier frequency ft: to form a bifurcatedend, a bifurcated conductive sleeve in capacitive relation to saidbifurcated end of said intermediate conductor, a conductive tube incapacitive relation to said inner conductor, a connection from saidconductive tube to one side of said bifurcated sleeve at the open endthereof, means coupling unmodulated carrier frequency energy f0 acrosssaid inner and said intermediate conductors, additional means coupling asource of offset frequency energy f0 between said outer conductor andsaid bifurcated sleeve, two nonlinear devices respectively connected todifferent sides of said bifurcated sleeve; a second mixer circuitincluding an inner conductor, an intermediate conductor surrounding saidinner conductor, an outer conductor surrounding both said innerconductor and said intermediate conductor, said intermediate conductorbeing slotted for a distance of approximately one-quarter wavelength atthe carrier frequency f0 to form a bifurcated end, a connection fromsaid inner conductor to one side of said bifurcated end of saidintermediate conductor, means coupling said signal modulated carrierfrequency wave fcim across said inner and intermediate conductors, aconnection from each of said two nonlinear devices of said first mixerto said intermediate conductor of said second mixer, two additionalnonlinear devices being respectively connected to different sides ofsaid bifurcated end of said intermediate conductor of said second mixer;and an output circuit coupled to said additional nonlinear devices andsaid outer conductor to derive a rectified product of said second mixer.

7. A balanced mixer comprising, inner, intermediate, and outer coaxialconductors, said intermediate conductor being slotted at one end to formtwo parallel conductors, means coupling one end of said inner conductorto the end of one of said two parallel conductors, means to couple afirst signal across said inner and said intermediate conductors, meansto couple a second signal across said intermediate conductor and saidouter conductor, two non-linear devices respectively coupled to the endsof said two parallel conductors, and an output circuit coupled to saidnon-linear devices to derive a rectified product of said mixer.

8. A balanced mixer comprising, inner, intermediate, and outer coaxialconductors, said intermediate conductor being slotted at one end to formtwo parallel conductors, means coupling one end of said inner conductorto the end of one of said two parallel conductors, means to couple afirst signal across said inner and said intermediate conductors, meansto couple a second signal across said intermediate conductor and saidouter conductor, two non-linear devices respectively coupled to the endsof said two parallel conductors, and an output circuit coupled to saidnon-linear devices to derive a rectified product of said mixer, saidoutput circuit including another coaxial line having an inner conductorcoupled to both of said non-linear devices, and having an outerconductor connected to said first outer conductor.

9. A balanced mixer comprising, inner, intermediate, and outer coaxialconductors, said intermediate conductor being slotted at one end to formtwo parallel conductors, means coupling one end of said inner conductorto the end of one of said two parallel conductors, means to couple afirst signal across said inner and said intermediate conductors, meansto couple a second signal across said intermediate conductor and saidouter conductor, two non-linear devices respectively coupled to the endsof' said two parallel conductors, and an output circuit coupled to saidnon-linear devices to derive a rectified product of said mixer, saidoutput circuit including another coaxial line extending at right angleswith said three coaxial conductors, and having an inner conductorcoupled to both of said non-linear devices,

and having an outer conductor connected to said first outer conductor.

10. A balanced mixer comprising, inner, intermediate, and outer coaxialconductors, said intermediate conductor being slotted at one end to formtwo parallel conductors, means coupling one end of said inner conductorto the end of one of said two parallel conductors, means to couple afirst signal across said inner and said intermediate conductors, meansto couple a second signal across said intermediate conductor and saidouter conductor, two non-linear devices respectively coupled to the endsof said two parallel conductors, and an output circuit coupled to saidnon-linear devices to derive a rectified product of said mixer, saidoutput circuit including two output conductors coupled respectively tosaid two non-linear devices, and a hollow conductor constituting anextension of said first outer conductor surrounding said two outputconductors.

References Cited in the file of this patent UNITED STATES PATENTS2,469,222 Atwood et al. May 3, 1949 FOREIGN PATENTS 654,523 GreatBritain June 20, 1951 OTHER REFERENCES Electronics, March 1947, page209.

